The invention relates to an apparatus for feeding flat items, e.g. (large) letters or cards, to a suction-type separating arrangement according to the preamble of claim 1.
In separating apparatuses which are intended for flat items and, from a stack of flat items which is positioned on a more or less horizontally arranged conveying belt or chain as the transporting means, withdraw individual items from the end of the stack, it is customary for the stack, which becomes shorter and shorter as a result of the items being withdrawn, to be fed to the separating apparatus (VE). These VEs use, inter alia, friction technology. The contact pressure of the first item was achieved here by the stack behind it pressing the first item onto the withdrawal element/s (AE). A frictional force acting in the withdrawal direction thus built up as a result of the normal force acting between AE and item. The disadvantage, however, was that this normal force also acted between the first and second items. The apparatus thus only functions reliably as long as the coefficient of friction between AE and item is greater than between the first and second items. Otherwise, the first and second items may be withdrawn together (double withdrawal). VEs with relatively high throughput requirements, meanwhile, use friction/suction technology in order to achieve optimum separating rates in respect of quality and quantity. The system of these friction/suction-type separators (RSV) makes it possible to resolve this contradiction since the necessary normal force is produced by the additional attachment by suction, and the advancement means then merely has the task of conveying items up to the VE. The normal force between the item which is to be withdrawn and the following item can approach zero as far as the withdrawal operation is concerned.
For optimum throughput, it is necessary, once one item has been withdrawn, for the next item to be available at the VE as quickly as possible without the abovementioned effects occurring. Up until now, for this purpose (see FIG. 2), use has been made of apparatuses which sensed the force of the stack 4 positioned against them and thus, in the case of the force decreasing or being absent, set the advancement mechanism, e.g. a circulating conveying belt 5, in operation (DE 196 12 567 C2). The items to be withdrawn were thus pressed against force-sensing elements, e.g. spring-loaded levers 3. Two of these levers are provided for each withdrawal arrangement, in such a manner that they can be moved independently of one another. Furthermore, you are evaluated via at least one sensor 9 (displacement or pressure).
In order to avoid the inclined positions detrimental to RSVs (lack of planar position against suction element=loss of negative pressure=less frictional force), these levers are evaluated with an AND connection, i.e. the advancement means is only switched off when both levers give the logical signal “active”. The disadvantage here is that the force produced by these levers acts in the same direction as the normal force which is necessary for withdrawal purposes. An obvious advantage of the RSV is relinquished again here. This is disadvantageous particularly in the case of items such as letters and flat items, since the forces which are necessary for optimum stack sensing are approximately in the same order of magnitude as the normal forces at withdrawal. The levers 3 force the items of the stack 4 away from the withdrawal means 1 (suction belt). The suction force thus always has to overcome the lever force. The levers 3 act statically as a brake between the withdrawal system and the item which is to be withdrawn and reduce the frictional force during the withdrawal process. Furthermore, at the moment at which the item is attached to the frictional elements by suction, the lever 3, rather than “measuring” the stack 4, only “measures” the item butting against it. It is only when the item has been drawn out of the lever mechanism that the lever 3 can sense the stack 4. For a high withdrawal capacity, the advancement system has to be very quick and/or integrated over a number of cycles.